This invention relates to a circuit substrate comprising nitride type ceramics, and it is concerned with an AlN substrate comprising a metallized conductive layer formed on its surface with a high joint strength.
Ceramic substrates are widely used in circuit substrates, and recently used in circuit substrates for use in hybrid integrated circuits are substrates comprising a sintered body of nitride type ceramics, as exemplified by a sintered body of aluminum nitride (AlN), having a high thermal conductivity, an excellent heat dissipation property and an electric insulating property. The nitride type ceramics are prepared generally in the following manner: A given amount of a sintering aid such as Y.sub.2 O.sub.3, Sm.sub.2 NO.sub.3 and CaO is blended in nitride type ceramic powder with optionally further addition of an acrylic resin binder to thoroughly mix the whole, and the resulting mixture is, for example, pressure-molded to make a sintering precursor (a formed product) having a given shape, followed by sintering at a given temperature in, for example, a nitrogen atmosphere.
Incidentally, in instances in which nitride type ceramic sintered sheets are used as substrates for semiconductors, it is necessary to form a conductive layer on the face on which a semiconductor is mounted. Hitherto, this layer comprises a metallized conductive layer of copper (Cu), gold (Au) or silver-palladium (Ag-Pd) formed on the surface of a nitride type ceramic sintered sheet by applying a DBC process (direct bond copper process) or a thick film process.
However, this conventional substrate has the following problems.
A first problem is that the joint strength between the above metallized layer thus formed and the surface of the nitride type ceramic sintered sheet is so low that a peeling phenomenon may often occur.
A second problem is the problem that may be caused when a certain semiconductor device or wire is brazed on the metallized layer thus formed thereon. More specifically, in the instance of the brazing for example, it is carried out at a temperature of about 800.degree. C. or so in a hydrogen-nitrogen mixed gas. Since, however, the temperature used at the time of the baking treatment for the above metallized layer is usually a lower temperature of about 600.degree. to 1,000.degree. C., the joint strength between the metallized layer and the surface of the nitride type ceramic sintered sheet is extremely lowered at the time of this brazing to make it virtually impossible to carry out the brazing. A third problem is the problem based on the difference in thermal expansion coefficient between the nitride type ceramic sintered sheet and the metallized layer. More specifically, like the instances of the brazing and the high-temperature soldering, the substrate mounted with a semiconductor device such as a silicon wafer experiences a severe heat cycle of heating/cooling when used. As a result, thermal stresses based on the difference in thermal expansion coefficient between layers are produced at the respective joint areas between the nitride type ceramic sintered body/metallized layer/brazed layer (or soldered layer)/semiconductor device, causing the action of peeling them.
In the case of the above metallized layer, the thermal expansion coefficient is greater as much as about 2 to 4 times than that value of the nitride type ceramic sintered sheet (thermal expansion coefficient: about 4.6.times.10.sup.-6 /.degree. C.) and is of a value of about a half of the value equivalent to the brazed layer (or soldered layer), showing a great difference from that of the nitride type ceramics. Accordingly, at the time of the heat cycle, fine cracks are liable to be produced at the interface between the metallized layer or brazed layer and the nitride type ceramics. Such fine cracks may gradually grow with repetition of heat cycles, finally sometimes bringing about the peel of the mounted semiconductor device.
Such problems are inconvenient as there may be lowered the reliability of an apparatus actually provided with the substrate mounted with a semiconductor device.
A fourth problem is that the joint strength between the above metallized layer and the nitride type ceramic sintered sheet is so small as to make low the reliability, like the second problem, when used at high temperatures.
For the reasons as above, it is recently practiced, in order to form the conductive layer, to form the metallized layer by coating on the surface of a ceramic substrate a paste chiefly comprised of molybdenum (Mo) or tungsten (W) and also titanium (Ti), zirconium (Zr) or hafnium (Hf) added thereto as an activation metal, followed by firing. In the resulting metallized layer, molybdenum and tungsten are thermoresistant metals that may not be oxidized by heat at the time of the firing, and titanium, zirconium and hafnium may act as the activation metal and adhere the thermoresistant metal on the surface of the ceramic substrate through the reaction with the ceramic.
Accordingly, also in the circuit substrates employing the nitride type ceramic substrate, there has been taken a method in which a paste for the metallized layer, chiefly comprised of the above molybdenum or tungsten and also an oxide or nitride of titanium, zirconium or hafnium added thereto as an activation metal is coated, followed by firing and reduction to form a conductive layer, or a method in which this paste is coated on the ceramic substrate, followed by firing to adhere the metallized layer thereon by the action of the activation metal, or the like method.
In the instance in which the metallized conductive layer comprises the above W layer, Mo layer or TiN layer, the above problems becomes less serious than the instance in which it comprises the Cu layer, Au layer or Ag-Pd layer previously mentioned, and it can be achieved to improve the joint strength to the nitride type ceramic sintered body and the reliability in a practical use.
However, the problems as follows have arisen in the circuit substrate on which the metallized layer is formed like this by coating, on the surface of the ceramic substrate comprised of the nitride type ceramic sintered body, a paste chiefly comprised of molybdenum or tungsten and also titanium, zirconium or hafnium added thereto as an activation metal, followed by firing to form the metallized layer.
That is to say, there may arise scattering in the joint strength of the metallized layer to the nitride type ceramic substrate, resulting in non-uniformity in the joint strength of the metallized layer. Also, the surface of the metallized layer is applied with plating in order to mount a semiconductor device on the metallized layer. In this occasion, however, it is sometimes impossible to surely form a coating on the surface of the metallized layer, because grain boundary phases of the AlN substrate exude to the surface of the metallized layer. Thus, an anxiety is accompanied when a semiconductor device is firmly fitted on the metallized layer.
In addition, in the method employing the metallizing paste obtained by adding and mixing a nitride of the above activation metal in the high-melting metal such as Mo and W, no sufficient joint strength can be attained unless the firing temperature is raised to higher temperatures as much as 1,700.degree. C. to 1,800.degree. C., so that warpage of the substrate may occur. Also, no metallized layer can be formed unless the activation metal is added in a large amount as much as 70% by weight or more, and therefore, because the amount of the high-melting metal to be blended becomes small, the electric resistance becomes greater and moreover the difference in thermal expansion coefficient from that of the ceramic substrate becomes greater. Thus, there have been involved the problem that cracking is liable to occur by the application of cold and heat cycles. Another problem is also caused such that the scattering in the joint strength also occurs in regard to the AlN ceramic substrates of about 150 W/m.multidot.k having many grain boundary phases.
In the method in which the metallizing paste employing the compound such as Mo and W, there have also been involved the problem that bleeding occurs at the time of the coating or firing to make it difficult to retain the shape of circuit patterns, or no sufficient surface properties.